This invention relates to imaging systems employing a migration imaging member which includes a substantially conductive substrate, having contiguous one surface thereof a layer of substantially electrically insulating softenable material, the softenable layer containing a layer of migration material comprising selenium-containing particles spaced apart from the softenable layer-substrate interface, as more fully described in portions of U.S. Pat. No. 3,975,195, the entire disclosure of which I hereby expressly incorporate herein by reference.
In one of the methods of imaging such a system, the imaging member is charge-sensitized, exposed to a light and shadow image pattern, and then developed or softened by contact with a liquid or vaporous solvent for the softenable material, or by heat, as a result of which migration material imagewise migrates at least in depth in the softenable layer. The various imaging materials and image processing steps useful in this type of migration imaging are fully outlined in U.S. Pat. No. 3,975,195.
One of the preferred migration imaging members comprises a thin layer of particles containing selenium located at or near one surface of a softenable layer of substantially electrically insulating material, itself residing on a conductive substrate, such as aluminized Mylar. Although imaging is obtainable with such a member irrespective of the polarity of charge sensitization or the method of softening development, for positive polarity charging, best images are obtained (that is, lowest background) where development is by vapor or liquid softening and wash-away; for negative polarity charging, heat softening is generally employed for best results.
There are, however, several reasons for desiring liquid development of negatively-charged imaging members. First, liquid solvent development produces the best image contrast, since the background is completely removed by the solvent. However, with heat development, the background remains on the member and the image is read-out by the difference in color between the migrated areas and the unmigrated areas (red versus blue). Second, although it has been found that latent electrical images of both polarity types can be set and stored for extended periods of time, negative set images have the longest demonstrated shelf life, and can be set simply by storage in the dark; positive images, however, must be set by pre-softening with liquid solvent, an inconvenient procedure. Negative latent images can also be set by a gentle presoftening by heat short of development. In this context, the word "set" is defined to mean extended life against light, and/or life against time. A fuller description of this phenomenon is contained in any of copending applications, Ser. Nos. 349,505; 349,506; and 349,585; all filed Apr. 9, 1973. It is disclosed in those applications that negative-charge latent migration images, is presoftened by heat short of development, or if stored in the dark for more than about 200 minutes, become light proof and can be developed in the light without destruction of the latent image, and can be stored indefinitely. Third, for liquid development of migration images, a novel system has been invented for continuously and rapidly developing a strip of migration imaging film, as disclosed in U.S. Pat. No. 3,878,816, by passing the strip through a nip of developing fluid. Such a system possesses obvious advantages.